Shop OBEX P1 Docs P2 Docs Learn Events
high-side switch — Parallax Forums

high-side switch

TomHTomH Posts: 3
edited 2007-10-31 17:30 in Propeller 1
Hello. When using a uC to switch power to a load, we normally can use an n-channel mosfet between the load and ground. But in some cases (servo, analog) the switch wants to be on the high side. Generally the load's supply voltage may be above the uC's. I am looking for recommendations for high-side switches. The simplest home-made version is an nmos driving a 10k resistor which drives a pmos. We do this all the time, but we'd love to find these three components in a single 3-pin 0.1" package that is as easy to use as an NMOS. Beyond that, there are high-side switch devices for sale which have many more pins and offer many unnecessary features (thermal shutdown, adjustable current limit, n-channel with Vgs charge pump) and are hard to find in a breadboardable package.
My dream part looks like: 3 pins (enable, vcc = 3-24V, and gnd); TO-220 or other 0.1"-pitch through-hole; enable logic threshold 1 or 2 volts; R_on < 0.1 ohm; Imax = 3 or 4 A.
We want to have tons of these in a handy bin for frequent use in prototypes or production.
How do others do this? Thanks in advance!

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-10-29 20:09
    If you need to have the grounds commoned between the uC and switched circuitry, high-side switching can never be achieved with a three-pin device. It will take four pins, minimum: Gnd, SignalInp, V+, and V+(Switched).

    Now, having said that, you could make Vdd and V+ common, leaving V- several volts below Gnd. In this case a simple P-channel MOSFET will do the trick.

    -Phil
  • TomHTomH Posts: 3
    edited 2007-10-29 20:13
    Ooops - Of course, FOUR pins: Enable, Gnd, Vcc, and Vout(switched). I have a six-month-old and am in a perpetual deer-in-headlights state. Yes, I would like to have a common ground point.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-10-29 20:49
    Something like the BTS409 may be what you're after. It comes in a TO220-5. DigiKey has other, similar high-side switches (search: high side on their website, without the hyphen).

    -Phil

    P.S. Caring for a six-month-old and merely dazed? I'd be catatonic! shocked.gif
  • deSilvadeSilva Posts: 2,967
    edited 2007-10-29 21:08
    ... my dream of a simple quadrupol tongue.gif
    749 x 452 - 47K
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-10-29 21:41
    It's messy on the inside to make it simple on the outside!
  • Graham StablerGraham Stabler Posts: 2,510
    edited 2007-10-29 21:43
    That dotted line could be a diecast box[noparse]:)[/noparse]
  • Fred HawkinsFred Hawkins Posts: 997
    edited 2007-10-31 05:21
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2007-10-31 17:01
    That BTS409 really does have everything but the kitchen sink!

    Another option is a small opto-relay, such as:
    http://www.irf.com/product-info/datasheets/data/pvdz172.pdf
    That particular one comes in a 4 pin DIP8 package and is rated for up to 1.5 amp.
    It has the advantage that the output is highly isolated (4000 volts) from the input, and it can switch up to 60 volts. A disadvantage is that the driver has to supply around 10 ma for the input LED to turn on the HEXFET.

    A third option is a low dropout voltage regulator with a shutdown input.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Tracy Allen
    www.emesystems.com
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-10-31 17:30
    Tracy,

    The VR option is a good one — especially with an adjustable regulator like the LM2941. You can just ground the feedback pin and have it fully on or fully off, depending on the enable input. Plus you get many of the protections available in the BTS409 at a reduced cost:

    ····BTS409: 2.3A switching, $172.00/C
    ····LM2941: 1.0A switching, $99.70/C

    Both come in a TO220-5 which, although it's a throughhole part, is hard to breadboard since the pins are so close together.

    I've used the PVDZ172s in products before. They're quite nice, and the DIP is easy to breadboard, but you pay for the isolation: $526.92/C from DigiKey.

    -Phil
Sign In or Register to comment.